Various blood processing systems now make it possible to collect particular blood constituents, rather than whole blood, from donors. Typically, in such systems, whole blood is drawn from a donor, the particular blood component or constituent is removed and collected, and the remaining blood constituents are returned to the donor. By thus removing only particular constituents, less time is needed for the donor's body to return to normal, and donations can be made at more frequent intervals than when whole blood is collected. This increases the overall supply of blood constituents, such as plasma and platelets, made available for health care.
Whole blood is typically separated into its constituents through centrifugation. This requires that the whole blood be passed through a centrifuge after it is withdrawn from, and before it is returned to, the donor. To avoid contamination and possible infection of the donor, the blood is preferably contained within a sealed, sterile system during the entire centrifugation process. Typical blood processing systems thus include a permanent, reusable centrifuge assembly containing the hardware that spins and pumps the blood, and a disposable, sealed and sterile fluid processing assembly that actually makes contact with the donor's blood. The centrifuge assembly engages and spins the fluid processing assembly during a collection procedure. The blood, however, makes actual contact only with the fluid processing assembly, which is used only once and then discarded.
As the whole blood is spun by the centrifuge, the heavier components, such as red blood cells, move outwardly away from the center of rotation toward the outer or "high g" wall of a separating chamber included as part of the fluid processing assembly. The lighter components, such as platelet-rich plasma, migrate toward the inner or "low g" wall of the separating chamber. Typically, an intermediate layer of white blood cells forms an interface between the platelet-rich plasma and red blood cell components of the whole blood during centrifugation. Various ones of these components can be selectively removed from the whole blood by forming appropriately located channeling seals and outlet ports in the separating chamber of the fluid processing assembly. Proper separation requires, however, that the interface between the platelet-rich plasma and the red blood cells be located within a particular zone between the high g and low g walls of the separating chamber. Displacement of the interface from the desired location can result in low platelet yield if the interface is too near the high g wall, or can result in a high whole blood cell count in the extracted plasma if the interface is located too near the low g wall. Good platelet yields along with low whole blood cell counts are achieved when the interface is maintained at the proper, desired location.
Various known centrifuges, such as those shown and described in U.S. Pat. No. 5,316,667, are operable to automatically keep the interface within the desired zone as the centrifuge operates. Typically, the separating chamber of the fluid processing assembly is loaded between the bowl and spool of a centrifuge. A ramped surface formed on the interior outer wall of the bowl deflects the high g wall of the separating chamber inwardly relative to the axis of rotation. The interface between the generally dark, opaque whole blood cell layer and the generally light, clear plasma layer appears as a line projected onto the ramped surface. Where, exactly, the line appears on the ramped surface is a function of the position of the interface between the high g and low g walls of the separating chamber. Accordingly, the position of the line on the ramped surface can be used to gauge the position of the interface between the high g and low g surfaces.
Automatic control over the location of the interface is achieved by sensing the position of the line on the ramped surface and thereafter adjusting the centrifuge operating parameters to place and keep the line within desired limits. In particular, by controlling the rate at which platelet-rich plasma is withdrawn from the separating chamber, the line can be "moved" up or down on the ramped surface. Typically, an optical sensor is used to sense the position of the line on the ramped surface. As the centrifuge spins past the sensor, the sensor develops an electrical pulse having a width related to the position of the line on the ramped surface. As the line moves closer to the high g wall of the separating chamber, the pulse width increases. As the line moves closer to the low g wall, the pulse width narrows. By sensing the width of the pulses developed by the optical sensor and thereafter using the pulse width to increase or decrease the rate at which platelet-rich plasma is withdrawn from the separating chamber, the line can be kept within desired positional limits on the ramped surface.
Prior, optically based interface detection and control systems responded only to the width of the pulse developed by the optical sensor and assumed that pulse width alone was a reliable indicator of the position of the interface line on the ramped surface. However, experience has shown that a variety of abnormalities or unusual operating conditions can arise that make pulse width, by itself, an unreliable sole indicator of proper interface positioning. For example, unusually high or low platelet counts in the donor's blood can change the light transmisitivity of the plasma and thus the apparent width of the detected pulses with no real change in the actual position of the interface line on the ramped surface. Similarly, a temporary accumulation of platelets on the ramped surface can cause a change in pulse width with no real change in the interface line position. Nevertheless, prior systems, which responded only to the width of the pulses developed by the optical sensor, would view either occurrence as requiring corrective action. The result would be to shift the interface away from the optimum position.